1. Field of the Invention
The present invention relates to a solar cell module and a process for producing said solar cell module. More particularly, the present invention relates to a solar cell module having a photovoltaic element module in which a plurality of photovoltaic elements (solar cells) are electrically connected with each other and a process for producing said solar cell module.
2. Related Background Art
In recent years, societal consciousness on the preservation of energy resources and the protection of environment has been increasing all over the world. Particularly there has been pointed out a fear for exhaustion of petroleum and the like as the energy resources. In addition, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO.sub.2 has been predicted.
In view of this situation, solar cells (photoelectric conversion devices in other words) are greatly expected to be a future power generation source since they can directly convert sunlight which is a clean energy source into an electric power and supply said electric power without causing CO.sub.2 buildup in the case of thermal power generation.
There have been proposed a variety of solar cells for commercial and home appliances. These solar cells include solar cells in which crystalline series silicon materials are used (these solar cells will be hereinafter referred to as crystalline series solar cell) and solar cells in which amorphous silicon materials are used (these solar cells will be hereinafter referred to as amorphous silicon solar cell). The amorphous silicon solar cell is more advantageous in comparison with the crystalline series solar cell because the semiconductor photoactive layer of the former which is composed of an amorphous silicon material (a--Si) can be easily and continuously formed in a large area and in a desired form by a relatively simple process, where the energy consumed in the formation of the semiconductor photoactive layer is relatively small, and therefore, the amorphous silicon solar cell can be produced at a reasonable production cost. In addition, for the amorphous silicon solar cell, its element thickness is as thin as about 1/1000 of that of the crystalline series solar cell and therefore, the cost of a raw material used in the production of the amorphous silicon solar cell is desirably low. And the amorphous silicon solar cell can be readily designed into a solar cell module in a desired configuration which can be used as a power generation source.
Such amorphous silicon solar cell comprises a photovoltaic element which typically comprises a back reflecting layer, a semiconductor photoactive layer comprising an amorphous silicon material and a transparent electrode layer stacked in this order on a substrate such as a glass substrate or a flexible substrate. In the case of using a flexible substrate, there are advantages such that film formation thereon can be readily and continuously conducted and the resulting photovoltaic element is light and excels in flexibility. The use of such photovoltaic element having these advantages enables one to produce a light weight solar cell module. In this connection, a proper flexible member is often used as the substrate in the production of an amorphous silicon photovoltaic element.
The production of a solar cell module using such amorphous silicon photovoltaic element is usually conducted in a manner of providing a plurality of amorphous silicon photovoltaic elements, electrically connecting the amorphous silicon photovoltaic elements with each other into a photovoltaic element module, and sealing the photovoltaic element module by means of a sealing resin. Thus, there can be readily produced a desirable amorphous silicon solar cell module which is light weight and has flexibility at a reasonable production cost. In view of this, in recent years, an amorphous silicon solar cell module having such advantages as above described has been positively installed on a roof or wall of a building. In this case, the solar cell module is used as a building material by laminating a reinforcing member on its non-light receiving side (that is, the side opposite the light receiving side). By laminating the reinforcing member in this way, the physical strength of the solar cell module is improved and the solar cell module is prevented from being warped or distorted due to changes in the environmental temperature.
In order for such solar cell module to receive sunlight as much as possible, it is installed on a sunny place such as a roof of a building in many cases. In the case where the installation place is a building's sunny roof having a large area, there is employed a manner of arranging a plurality of such solar cell modules such that they are electrically connected with each other into a solar cell array.
By the way, in the process of mass-producing a plurality of photovoltaic elements, it is difficult to make all the photovoltaic elements produced be uniform in terms of property, where they are usually varied with respect to their properties and attributes. As related factors of causing this situation, there can be mentioned a variation in the thickness of the semiconductor photoactive layer, a deviation in the condition of forming the semiconductor photoactive layer, and a difference in the film-forming position upon forming the semiconductor photoactive layer, and the like.
As above described, in the case of producing a solar cell module, a plurality of given photovoltaic elements are electrically connected with each other. The electric characteristics of the resulting solar cell module depend on the properties of the respective photovoltaic elements. Here, in order to ensure the output of a solar cell module, there are presently prescribed standards for the output value of a photovoltaic element and that of a photovoltaic element module. Therefore, said photovoltaic elements and said solar cell module are generally produced in accordance with the above described standards.
Now, in the case of a solar cell module or a solar cell module string (which will be occasionally simply called "string" in the following) in which a plurality of photovoltaic elements are used as a unit in which these photovoltaic elements are electrically connected in series or parallel connection, when said photovoltaic elements are different from each other in terms of Pmax point, all the photovoltaic elements cannot be used at the same Pmax point. Therefore, a loss is unavoidably caused upon outputting an electric power. Similarly, in the case of a solar module array (which will be occasionally simply called "array" in the following) in which a plurality of solar cell module strings are electrically connected with each other in parallel connection, when these strings are different from each other in terms of Vpm (Pmax point voltage), it is a matter of course that all the strings cannot be used at the same Pmax point. In this case, a loss is also unavoidably caused upon outputting an electric power.
In order to prevent such drawback as above described from occurring, in the process of mass-producing a number of photovoltaic elements, it is required to constantly conduct the process control with circumspect care so that a variation does not occur among the photovoltaic elements obtained with respect to their properties. In this case, the period of time and the expenses required in order to conduct the process control in such a way as above described are of an extent which cannot be disregarded.
In the case where the photovoltaic elements obtained are such that some of them have different properties from those of the remaining photovoltaic elements, in many cases, these photovoltaic elements having different properties are treated, for instance, as will be described below.
(1) The photovoltaic elements having different properties are discarded. This situation results in a decrease in the yield.
(2) From the photovoltaic elements having different properties, the photovoltaic elements having the same properties are selected, and using these selected photovoltaic elements, a particular solar cell module array is prepared.
In any of the above treatments (1) and (2), extra expenses and extra treatment steps are necessitated. This situation results in an increase in the production cost of the solar cell modules as a whole.
In the case of continuously producing a photovoltaic element module by electrically connecting a plurality of photovoltaic elements, photovoltaic elements produced are sequentially connected in the order as produced. In this case, each adjacent photovoltaic elements situated next to each other are those produced in lots which are quite similar in terms of conditions relating to the film formation and film thickness. Particularly, the photovoltaic elements which are quite similar with respect to their properties and attributes are electrically connected with each other. In this case, although wastage at an element level is decreased, a variation among the photovoltaic elements remains to reflect a variation among photovoltaic element modules produced.
In the case of producing a number of photovoltaic element modules in such a way as above described, the resulting photovoltaic element modules are varied in terms of characteristics to a relatively large extent. For instance, in the case of producing a plurality of photovoltaic element modules by electrically connecting only the photovoltaic elements having inferior properties and attributes, the resulting photovoltaic element modules are liable to have an output value which is lower than the prescribed output value. This situation results in a decrease in the yield.
Further, there is an occasion of causing a problem in view of design. Particularly, the color tone of a photovoltaic element is based on interference colors of its constituent layers, i.e., the transparent electrode layer, reflection preventive layer, semiconductor photoactive layer, back reflecting layer, and the like. It can be said that when given photovoltaic elements are varied with respect to their constituent layers' thickness, they have a different color tone. And when a plurality of photovoltaic element modules are produced by using these photovoltaic elements, not only their insides are different from each other but also their color tones are different from each other. For solar cell modules produced by resin-sealing these photovoltaic element modules, as a matter of course, have a different color tone.
By the way, for building materials used for forming a roof or wall, they are required to have the same color tone in view of design. For roofing steel members, in general, they are required to have a chrominance which is within .DELTA.E=1. In the case of an ordinary building material whose color tone is decided depending upon the kind of a varnish used, it is possible to diminish the extent to which its appearance is mottled in accordance with a manner of coating a varnish or by using a proper varnish. But in the case of a solar cell module, it is not appropriate for its light receiving face side to be colored because of leading to decreasing the photoelectric conversion efficiency.
Now, Japanese Unexamined Patent Publication No. 217446/1997 discloses a wall face structure of a building in which even when panel members are different from each other in terms of finishing such as coloring, a wall face formed by connecting these panel members is free of a mottled appearance. Particularly, in the wall face structure described in this document, the panel members are curved, and they are connected through their side edges in the longitudinal direction so that rays irradiated are irregularly reflected by the respective panel members to avoid a mottled appearance.
However, according to the structure described in the above document, it is required for building materials to be curved. Therefore, it is necessary for the photovoltaic elements used in a solar cell module to be also curved. In this case, it is difficult to ensure the reliability of the solar cell module. And in the case of a photovoltaic element comprising a crystalline silicon material or a solar cell module having a glass plate on the outermost surface on the light receiving face side, it is difficult for them to be processed into a curved form.